When I started writing, I had the idea that it was correct to use two spaces after a period. However, after submitting my first manuscript for editorial advice, I learned that one space is preferred. I just saw this interesting blog on Slate, called Space Invaders. It discusses the issue with lots of links, including style manuals. It seems that typographers early in the last century decided to standardize and made the one-space rule. Apparently there was backsliding due to the typewriter, which used monospace type until the 1970’s. This gave every letter the same amount of space and made the text look ragged. Therefore, two spaces made the manuscript easier to read. But, says Slate, there is no longer any excuse for using two spaces after a period. I’m glad I went back and took all those extra spaces out, even though it was tedious at the time :).